Pulse-amplitude modulation (“PAM”) is a common mechanism for high-speed applications such as physical link layers, with PAM2 (also known as NRZ) being the most commonly used to support up to 28 gigabits per second (Gbps) for serial communications. More recently, PAM4, which uses four distinct amplitude levels (e.g., −1, −⅓, +⅓, +1 volts) each of which is represented by a combination of two bits (e.g., 00, 01, 10, 11) has been suggested for use in high-speed communication links beyond 28 Gbps.
To test a high-speed communication link, a test pattern may be transmitted by a transmitter over such a communication link for receiving and checking for errors by a receiver. Such test pattern may thus be used to determine impact of channel loss, intersymbol interference (e.g., crosstalk), and other factors which may have an effect on an error rate. A pseudo random binary sequence (“PRBS”) is commonly used as a test pattern for PAM2/NRZ. However, PRBS patterns when mapped to PAM4 could result in poor baseline wander and clock content. Others have suggested using a quaternary pseudo random binary sequence (“QPRBS”) to produce a test pattern for PAM4 to achieve similar random characteristics.
Hence, it is desirable and useful to provide an IC having a capability of generating a QPRBS and/or checking a communicated QPRBS.